BF: Crusader

AF (Adoptive Father) is diffrent from normal Big Daddy. Inside the armor is a Many (completely mutated being like Gil Alexander) from CC earlier experiment subject. Manies have lost human mind from splicing and mutation, making them are very aggresive and often release their savage on everything in their sight either human or Little Sister.

Almost of AF Series have been sealed their brain and sentiment, making them into a half dead/half slumber situation. AF function can operate through "Mental Bond" from Little Sister which connected AF body together with Little Sister nerve and mind, allowing bonded Little Sister to command her AF freely like her own body.

Although, this kind of bonding is more dangerous to Little Sister than normal Big Daddy bonding since they have been sharing their mind together. The damage that AF taken also harming to Little Sister's brain and mental directly. Therefore, the death of AF probably leads bonded Little Sister to become unconscious, lunatic or even dead.

AF seal can be broken if bonded Little Sister mind is too weak to control him. As a result, AF will turn into Berserk Mode and attack everything, both ally and enemy.

Ps. AF theory is quite similar to Evangelion.